1. Field of the Invention
The invention relates to a method for fabricating a BiCMOS device, more particularly to a method for fabricating a BiCMOS device in which the process steps involved with the formation of a base region of a bipolar transistor of the BiCMOS device are modified so as to attain better bipolar transistor characteristics.
2. Description of the Related Art
BiCMOS devices are devices which employ both bipolar and CMOS transistors in the same integrated circuit. Conventionally, BiCMOS devices are formed by forming the bipolar and MOS transistors in selected areas of the device and interconnecting the two types of transistors. This often results in a relatively complicated fabrication process because of the large number of process steps involved.
U.S. Pat. No. 5,013,671 (Havemann) discloses a method for manufacturing simultaneously both types of transistors while minimizing the number of process steps involved. In the above cited patent, a bipolar base region of a bipolar transistor is defined with the use of a masking layer. The masking layer exposes the intrinsic portion of the bipolar base region and does not mask a small tab region adjacent to the intrinsic region. A relatively thick base oxide layer is grown over the intrinsic base and tab regions, and the base dopant is implanted through the base oxide layer. An emitter region of the bipolar transistor may be formed from a doped polysilicon layer on the intrinsic base. When spacers are formed on each side of the polysilicon emitter of the bipolar transistor, the thickness of base oxide at the tab region is decreased. An extrinsic base and an overlap region are formed simultaneous with the formation of the source/drain of one of the MOS transistors of the BiCMOS device and have dopant concentrations which are higher than that of the intrinsic base.
A main drawback of the method described in the above mentioned patent is that control of the base profile of the bipolar transistor is difficult to achieve because the base dopant is implanted through the thick base oxide layer, and further because the base dopant implant step is followed by a thermal cycle for a gate oxide of the MOS transistors.